When a three-phase brushless motor (hereinafter, referred to as a “three-phase motor”) is driven, a PWM-control has been used in the related art. In the PWM-control, when the magnitude of a current flowing through the three-phase motor (motor drive current) suddenly changes, a motor torque also suddenly changes, and thus in some cases, a noise occurs according to a sudden change of the motor torque. Therefore, a technique of suppressing the noise has been examined (e.g., JP2004-032953A (Reference 1)).
In a motor drive device described in Reference 1, when a PWM-control is performed by a PWM signal, in a period before switching from a high impedance state to a low impedance output state, as a period less than ½ of a high impedance state section, and in a period after switching from the low impedance output state to the high impedance state, as a period less than ½ of a high impedance state section, a change of a motor drive current is gently made through a SWEEP control of a DUTY ratio, so that a noise reduction is executed.
As in the technique described in Reference 1, in the period before switching from the high impedance state to the low impedance output state or in the period after switching from the low impedance output state to the high impedance state, a SWEEP control of the DUTY ratio has not only an effect of noise reduction, but also an effect of suppressing a surge that occurs after switching from the low impedance output state to the high impedance state. Meanwhile, as a method of executing a noise reduction, a method of extending an energizing time from a 120-degree energization drive of the related art and approaching a 150-degree energization drive or sinusoidal wave drive has recently been employed. However, in a three-phase motor controlled in a sensorless manner, when an energizing time is extended from a 120-degree energization drive, a position detecting period during which a position of a rotor is detected becomes shorter, and further, the surge becomes larger, so that it is impossible to perform a stable control. It is possible to suppress the surge by using a SWEEP control of the DUTY ratio, but in some cases, it is difficult to obtain a sufficient effect depending on the rotation speed of the three-phase motor.
Thus, a need exists for a motor control device which is not susceptible to the drawback mentioned above.